Construct Plane: using 2 surfaces what is the best approach?

I have a datum that is 2 separate plane surfaces with opposing vectors but on the same center line.

What is the best approach: creating a few independent points on both surfaces and combining them into one plane OR creating 2 separate planes and then Constructing a plane from them?

R2ah1ze1l over 1 year ago

Point to point data is easier to review than plane averages vs plane averages. But that's just me, maybe.
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JesseB413 over 1 year ago

I would create two separate planes and then construct a 3D width from them.
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BIGWIG7 over 1 year ago in reply to JesseB413

why?.....................
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JesseB413 over 1 year ago in reply to JesseB413

Because that's the closest way to represent a datum with 2 plane surfaces. Constructing a center plane from the two would also work if that's what you would like to do.
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BIGWIG7 over 1 year ago in reply to JesseB413

i just performed a quick test using your method and the results using a width are much worse than using a conventional plane made out of points from each surface.

can you comment on your source for using a constructed width is coming from?

for a bit more info i am using this plane to clock, not to level.
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Paperback Rocker over 1 year ago

Width.

Rotate to it.
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BIGWIG7 over 1 year ago in reply to Paperback Rocker

again why? whats the reasoning? is pcdmis calculating the data better this way?
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NinjaBadger over 1 year ago

I'd say measure two planes.
create COP from two planes Construct secondary datum plane from the COP (I think you can do this but can't check right now.) I think this will give a resultant plane external to material, so if the planes aren't actually coplaner it should give the best simulated plane to represent assembly conditions.
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Cris_C over 1 year ago

I have a datum that is 2 separate plane surfaces with opposing vectors but on the same center line.

What is the best approach: creating a few independent points on both surfaces and combining them into one plane OR creating 2 separate planes and then Constructing a plane from them?

It is hard to say what is the best approach. I think it would depend a lot on the situation.

If you have callouts that use MMB on that datum, then I'd certainly suggest you use a width feature to take advantage of the bonus tolerance. Same goes for if there is MMC or LMC on the datum callout itself.

If you are using the newer GeoTol reporting. I think you would technically want to use a width feature because it will then be treated as a datum simulator. Under ASME It would use a constrained least squares fit on the points to sit on the high spots, as if you were using a functional gage. Under ISO, it would do the same thing, but filters out outliers first.

Check out the Width section of this page of the help file. There is lots of good info there: How PC-DMIS Solves and Uses Datums (hexagonmi.com).
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NinjaBadger over 1 year ago

I'm not sure why people are advocating a width in this example. I'm not saying it's wrong but from the description it doesn't sound like in terms of function it behaves as a width.

I'm not even sure if it would be a valid datum.

I'm picturing two nominally coplaner planes but facing opposite directions from each other, the fact they're nominally coplaner but facing opposite directions suggests to me they're offset from each other?
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